1. Field of the Invention
The present invention relates to an image heating apparatus suitable if used as a fixing apparatus mounted on a copying machine or a printer, and in particular, it relates to an image heating apparatus using a flexible sleeve.
2. Related Background Art
A description will be made with a fixing apparatus in an image forming apparatus such as an electrophotographic copying machine and a printer as an example. In general, in the image forming apparatus, as the fixing apparatus (a fixer), which heats and fixes an unfixed toner image on a recording material surface as a permanent fixed image indirectly (transferred) or directly formed and borne on a recording material (paper) by an adequate image forming process means such as the electrophotographic process, a heating apparatus of a heat roller system has been conventionally in wide use.
In recent years, an apparatus of a film heating system has been put into practical use in view of a quick start and energy saving. Further, the heating apparatus of an electromagnetic induction heating system, which allows a film composed of a metal to heat itself, has been proposed.
a) Fixing Apparatus of Film Heating System
The fixing apparatus of the film heating system, for example, has been proposed in Japanese Patent Application Laid-Open Nos. S63-313182, H02-157878, H04-44075, H04-204980, and the like. That is, a pressure welding nip portion (hereinafter referred to as fixing nip portion) is formed by nipping a heat resisting film (hereinafter referred to as fixing film) between a ceramic heater as a heating body and a pressure roller as a pressure member, and a recording material forming and bearing an unfixed toner image is introduced between the fixing film of the fixing nip portion and the pressure roller, and is nipped and conveyed together with the fixing film, so that the unfixed toner image is fixed on the surface of the recording material by an applied pressure of the fixing nip portion, while giving a heat of the ceramic heater through the fixing film.
This fixing apparatus of the film heating system can constitute an on-demand type apparatus by using a member of low heat capacity for the ceramic heater and the film, and may generate heat of a predetermined fixing temperature by turning on the ceramic heater of a heat source only when an image formation is performed, and has advantages in that a waiting time from turning on the power source of the image forming apparatus until reaching an image formation executable state is short (quick startability), and a power consumption during a standby time period is sharply reduced (power saving) and the like.
Further, in the publications of among the above described Japanese Patent applications, those described in Japanese Patent Application Laid-Open Nos. H04-44075 and H04-204980, the film (flexible sleeve) by driving the pressure roller (pressure roller driving type), and comparing to the type which allows the flexible sleeve to rotate by providing a driving roller and a tension roller in the interior of the flexible sleeve, have an advantage of being simple in the structure.
b) Fixing Device of Electromagnetic Induction Heating System
For example, in the Japanese Patent Application Laid-Open No. H07-114276, there has been disclosed an induction heat fixing device, in which an eddy current is induced in a metallic layer (heat generating layer) of a fixing film by magnetic flux, and heat is generated by its joule heat. This device can make the fixing film to directly generate heat by utilizing the generation of the induced current, and achieves a fixing process of higher efficiency than the fixing device of the heat roller system which has a halogen lamp as its heat source.
In FIG. 14 is shown a schematic structure of an example of the fixing apparatus of the electromagnetic induction heating system which has improved its efficiency by concentrating the alternating magnetic flux of an excitation coil into a fixing nip portion. This fixing apparatus comprises a fixing film (flexible sleeve) 10 having a metallic layer, a film guide member 16C disposed in the inner face of this film, a magnetic field generating means 15 having an excitation coil 18 and a magnetic core 17, and a pressure roller 30 allowing the film 10 to rotate. In this example, a nip portion N conveying a recording paper P is formed by the film guide member 16C and the pressure roller 30. When the excitation coil 18 is energized, an eddy current is generated in the metallic layer of the fixing film 10, and the fixing film generates heat. By this heat, a toner image t on the recording paper P is heated and fixed.
As described above, both of the fixing device of the system which heats the toner image by a heater through the flexible sleeve and the fixing device of the system which heats the toner image by heating the flexible sleeve itself use the flexible sleeve.
In such a fixing device using the flexible sleeve, particularly in the device using a flexible sleeve made of a metal, it is necessary to take into consideration a breakage of sleeve due to metal fatigue. Further, from among the devices using the above described flexible sleeve, the device of a pressure roller drive type is simple in structure, and moreover, a load given to the flexible sleeve can be made small. However, particularly in case the flexible sleeve made of a metal is used, even if it is the device of the pressure roller drive type, it is necessary to take into consideration a breakage of sleeve due to metal fatigue. For example, when the sleeve 10 is driven by the pressure roller 30, the sleeve 10 approaches one side in a generatrix direction, and in case its approaching force is strong, a buckling stress is generated in the end portion in the generatrix direction, thereby accelerating a fatigue phenomenon. Alternatively, since a large frictional force acts on the end portion by its approaching force, the sleeve 10 is unable to aptly slide on the surface against which the sleeve 10 abuts, and the sleeve 10 is locally over deformed, thereby accelerating the fatigue phenomenon.
Thus, the flexible sleeve made of a metal is prone to break, comparing to the flexible sleeve made of resin such as polyamide, and consequently, various designs have been made to consider and control the metal fatigue of the flexible sleeve.
Incidentally, the present inventors have found a new cause which invites the metal fatigue of the flexible sleeve.
This cause is such that, when the sleeve 10 is driven by the pressure roller 30, though the sleeve is deformed into a shape as natural as it desires, it is forced to be partially reformed into another shape, and as a result, the stress partially acting on the sleeve becomes large, so that the fatigue phenomenon is accelerated.